Several classes of steroid hormones (androgens, estrogens, and progesterone) were quantified by sensitive and specific radioimmunoassays in plasma from fetal rhesus monkeys (days 59-163 gestation). Male fetuses had significantly more testosterone in their circulatory system than females (1385 plus or minus 249 pg/ml, N equals 8 vs. 442 plus or minus 73 pg/ml, N equals 10). No sex differences were observed for androstenedione and dihydrotestosterone, an indication that testosterone is probably the morphogenic substance in the circulatory system. Evidence was obtained that the rhesus placenta produces principally estrone (E1) and that the small quantities of estradiol (E2) found early in gestation in the fetus are from the peripheral conversion of E1 yields E2. These data suggest that the level of dehydrogenase activity controls the level of E2 in the fetus. Other data on fetuses studied at later gestational ages (124-145) indicate that the fetal ovary biosynthesizes about six times more E2 than the fetal testes in vitro. Negative and "positive" feedback effects of E2 were studied in gonadectomized male, female, and pseudohermaphroditic rhesus monkeys. Estrogen facilitates the release of LH in all three groups. The quantity of release, however, depends upon the initial degree of suppression by E2. Thus males and pseudohermaphrodites which had less suppression with an equal quantity of E2, had larger LH surges and thus perhaps less sensitivity to E2 than females. Regardless, the effect of prenatal androgens on the differentiation of the neural centers that regulate gonadotropin secretion in primates is different from that in rodents.